In the bowl of a mixer, beat the softened butter and shortening until creamy.
Add in the meringue powder, confectioners' sugar, vanilla, and salt. Mix on low to combine.
With the mixer on low, stream caramel sauce and half of the heavy cream into the bowl and mix on medium-high. Add more cream (1 tablespoon at a time) until a medium-thick frosting consistency is reached.
Recipe by Her Modern Kitchen at https://www.hermodernkitchen.com/girl-scout-cookie-samoa-cake/